Polyamide acids may be defined as polymers formed by the polycondensation reaction of
A. AMINES HAVING TWO AMINO GROUPS, EACH OF WHICH IS PRIMARY OR SECONDARY IN NATURE, ATTACHED TO SEPARATE CARBON ATOMS, WITH
B. ORGANIC ACIDS (OR DERIVATIVES OF ORGANIC ACIDS CAPABLE OF FORMING AMIDES) CONTAINING THREE OR MORE CARBONYL GROUPS, AT LEAST TWO OF WHICH ARE ATTACHED TO ADJACENT CARBON ATOMS,
The conditions of the polycondensation being such that only two of the amide forming groups take part in the reaction, thereby yielding a polyamide having unreacted carboxylic groups pendent from the repeating units of the polymer chain.
One of the polymerization processes useful in the preparation of these polyamide acids is generally carried out in an organic liquid which is usually a solvent for at least one reactant and contains functional groups other than primary or secondary amino or carboxyl or anhydride groups. The process has been described for example in Edwards, U.S. Pat. No. 3,179,614, issued Apr. 20, 1965.
The polyamide acid compositions can be formed into useful structures by extruding through dies, casting as films, coating on substrates or by similar processing. They can be cured into polyimide polymers. The polyamide acid compositions in solvents can also be used as filled or pigmented coatings for a variety of substrates.
Polyamide acids, whether dissolved in organic liquids or without solvents, suffer from the disadvantage that when exposed to water or even moisture from the atmosphere they tend to undergo some hydrolytic degradation.
To overcome such hydrolytic instability, and to provide polyamide acids in solutions containing significant amounts of water, such polyamide acids are converted into salts with organic bases, preferably tertiary amines. If amines other than tertiary amines are utilized in the salt forming reactions with the polyamide acids, the hydrolytic instability is not totally overcome.
As practiced by the prior art, as for example in Reynolds, British Patent Specification No. 1,207,577, published Oct. 7, 1970, a tertiary amine such as trimethyl amine is utilized in approximately stoichiometric amounts corresponding to the polyamide acid. The amine salt of the acid is prepared in an aqueous alcoholic solution and films are cast from this solution. Such solutions, however, can only be prepared at low non-volatile content without becoming excessively viscous. Also, only very thin films can be obtained by this method of the prior art and film quality is often poor. The films prepared this way are often bubbly and opaque since the polyamide acid and the polyimide, generated during the film forming process through the loss of amine and water, are insoluble in the solution of the polyamide acid salt. These insoluble film portions blister during further solvent loss giving rise to imperfect, opaque films.
It would be advantageous to be able to prepare high solids, low viscosity stable aqueous solutions of polyamide acids. When dealing with organic solutions of these polyamide acids they have to be protected from atmospheric moisture as mentioned above. Also, the increased awareness of the environmental hazards of allowing organic solvent vapors to enter the atmosphere dictates the diminished use or elimination of such solvents wherever possible. It is also a practical necessity to have a solution as high in non-volatile content as possible and to obtain this goal within reasonable viscosity limits. This way it is possible for example to deposit films of a desired thickness without having to repeat this operation several times.
This invention affords stable aqueous solutions of polyamide acid precursors of polyimides at up to 70% solids content, at viscosities not exceeding 10,000 centipoises at 50.degree. C. This is accomplished by preparing solutions of polyamide acid salts of such acids with tertiary amines; wherein said solutions also contain a viscosity reducing agent such as furfuryl alcohol and a coalescing agent such as N-methylpyrrolidone. These solutions of the polyamide acid salts are capable of depositing uniform films curable to polyimide polymers.